The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)

          @misc{ pirsa_PIRSA:23100064,
            doi = {10.48660/23100064},
            url = {https://pirsa.org/23100064},
            author = {Buoninfante, Luca},
            keywords = {Quantum Gravity},
            language = {en},
            title = {The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)},
            publisher = {Perimeter Institute},
            year = {2023},
            month = {oct},
            note = {PIRSA:23100064 see, \url{https://pirsa.org}}
          }
          

Abstract

In the context of perturbative quantum field theory (QFT), the addition of quadratic-curvature invariants to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. This theory exhibits unusual features due to an additional massive spin-2 ghost which, in general, may cause instabilities. In the first part of this talk, we focus on the possibility of giving up locality as a way to avoid ghost-like degrees of freedom and provide a critical assessment on open questions in nonlocal theories of gravity, such as the uniqueness problem. In the second part of the talk, we take a step back and argue that, despite the presence of the ghost and actually thanks to it, Quadratic Gravity can still provide a consistent local perturbative QFT description of the gravitational interaction and explain new physics beyond Einstein's general relativity, e.g., it offers a natural explanation for the inflationary phase. Finally, we argue that a type of nonlocality in gravity can still occur non-perturbatively and show that a new lower bound on scattering amplitudes indicates that the gravitational interaction is intrinsically nonlocal if black holes form.